Yanmar Vio45 2003 excavator Hydraulic Oil filling

I need to top up the hydraulic fluid on my 2003 Yanmar Vio45 excavator. I don't have a user manual and the only one I can find online is for the later model, whose serial number begins at a high number than mine, which is 497xx (hiding the last two digits). The manual for the newer model has very different instructions for adding hydraulic oil. The tank is on the opposite side, for a start.

Below is a photo of the general area, hydraulic oil tank located at the top left. There's a large round cover with x3 12mm hex bolts. Another smaller cover with the flexible pipe coming from it and work x3 smaller hex bolts and then one large bolt beside that, which looks to be about 30mm in diameter, at a guess. The top of the photo is the rear side of the machine, for reference.

Would anyone know which is the correct part to remove in order to add the fluid?